If you run the compressor continuosly the lowest you'll get is about -14F. Doing this occasionally is probably ok as it's a feature on some models (Fast Freeze).

Yeah, even my good refrigerator has a feature like that called "Speed Ice". Drops the freezer down to 6 below so it can turn out batches of ice faster. It's pretty handy when we use up all the ice for a cooler or something. Makes ice cream a real bitch to scoop, though....